1. Field of Invention
The present invention relates to a method of forming a flash memory. More particularly, the present invention relates to a method of forming the shallow trench isolation (STI) structures and buried bit lines of flash memory.
2. Description of Related Art
Shallow trench isolation (STI) is an important technique for isolating small dimensional devices on a silicon chip. However, forming STI structures on a substrate with buried N+ bit lines is no easy operation. There are two main reasons:
1. The buried bit lines and the active regions are perpendicular to each other; and, PA1 2. If the buried bit lines are formed before etching out the trenches of STI structures, each continuous buried bit line will be cut into multiple segments.
Since the active regions and the buried bit lines are mutually perpendicular, the silicon nitride hard mask layer for patterning the active region and etching out trenches generally consists of a plurality of silicon nitride lines with portions of each buried bit line exposed. If the silicon nitride hard mask is used as an etching mask for producing the trenches, the buried bit lines will be cut into several discontinuous segments so that the original function of conducting an electric current is lost.